Any idea on this vintage JBL preamp?

[pyonc]

Some bleed through with pre-amps or integrated amplifiers which have singular controls affecting both channels, master volume pots or sliders, tone controls or any mechanical all in one mode or function selector switch is perfectly normal operation and is not in and of itself an indication of a defect or performance problem. Especially those with any sort of phase reverse or channel reverse switch, MONO/STEREO switches etc (talking vintage gear here) Especially when unbalanced inputs are used

And it's not always going to be balanced, the bleed through, in fact it seldom is

Verify my speculation quickly with a look at the pre-amp's schematic as well as the final outputs

I'm really not understanding your problem, how much (or better, how loud) is what you are hearing when you think you shouldn't? Is it sound being projected out into the room or something you have to put your ear up close to the speaker to hear?

......

Joe

Disappointing news from the tech who diagnosed this unit. He verified the signal bleed through the right channel, but couldn't find the so-called fade control that fits this specific SG520. It's obsolete, non-obtainable even from JBL. Returned to me not repaired, accordingly.
Now, I'm back to this thread, and appreciate very much your detailed and kind reply.
You asked: "Is it sound being projected out into the room or something you have to put your ear up close to the speaker to hear?"
My answer is 'audible bleed' (about 2 out of 10 in volume level) when I put my ear up close to the speaker on the right when I slide the volume knob all the way to the left.
When in stereo mode, however, I don't hear any audible imbalance between the left and right speakers.
I think I have to focus on listening to music from now on rather than worrying ...

[grumpy]

I think I have to focus on listening to music from now on rather than worrying ...

a healthy attitude!